A reed relay and a switch matrix device using such reed relays have been conventionally utilized to measure an electrical characteristic of one or more devices under test (DUT) by suitably switching the connections between plural measuring devices and plural devices under test (DUTs).
FIG. 1 is a partial view of an example of a measuring device using the matrix device as described above. This device includes a reed relay matrix rr.sub.jk (j, k=1, 2, 3) provided at each lattice point of the matrix. The matrix comprises signal lines 21 through 23 arranged in a lateral direction and signal lines 24 through 26 arranged in a longitudinal direction. In this device, a voltage of a D.C. power source 1 is applied to the DUT 2 by selecting a suitable combination of on-and-off states of the reed relays rr.sub.jk at the respective lattice points. In FIG. 1, only rr.sub.12 and rr.sub.21 are selectively switched on. A current flowing through a DUT 2 is measured by an ammeter 3 to analyze an electrical characteristic of the DUT 2.
In such a measuring device, a very small amount of current would be measured by the ammeter 3 if the DUT 2 had a high resistance. When a coaxial cable with grounded outer conductor is used for signal lines 21 through 26, the effect of an external electromagnetic field and a mutual electromagnetic field are substantially eliminated. However, because a potential difference occurs between the central conductor and the outer conductor, there occurs a problem that a leak current unfavorably flows through the insulating material between the central conductor (signal line) and the outer conductor of the coaxial cable.
As shown in FIG. 2, a conventional reed relay comprises a reed switch 11 which has both ends connected to the signal lines, a conductive cylindrical member (guard pipe) 13 which is disposed so as to cover the reed switch 11 over an insulating material 12 and a driving coil 14 which is wound around the peripheral surface of the guard pipe 13. The reed relay of such construction also has the same problem as the coaxial cable. That is, a detrimental leak current flows through the insulating material 12 between the signal lines of the reed switch 11 and the guard pipe 13, if the guard pipe 13 is grounded to eliminate the effect of the external electromagnetic field.
The leak current occurring in the coaxial cable can be prevented by equalizing the potential of the outer conductor with the central conductor. By connecting the outer conductor to a guard terminal, the potentials are kept at the same as the central conductor. Likewise,the leak current occurring in the reed relay can be prevented by equalizing the potentials of the signal line and the guard pipe 13 of each reed relay. For this purpose, lines constituting the grid of the switch matrix are classified into two groups; one is a signal line group, and the other is a guard line group for connecting the guard terminals to the guard pipes 13.
FIG. 3 is a partial view of a measuring circuit with the switch matrix device comprising the two line groups as described above. In FIG. 3, a D.C. power source 1 is connected through an ammeter 3 to a signal line 31 (shown in a lateral direction) and a guard line 32 (shown in the lateral direction). The signal line 31 can be connected to each of signal lines 33, 35 and 37 through the corresponding reed relays rr.sub.11, rr.sub.12 and rr.sub.13 as shown in FIG. 3. Guard line 32 is connected to guard pipe 13 as shown in FIG. 2 (not shown in FIG. 3) of each of the reed relays, and it can also be connected to each of guard lines 34, 36 and 38 through corresponding connecting switches sw.sub.11, sw.sub.12 or sw.sub.13. In FIG. 3, a current measurement of the DUT 2 is performed by closing only reed relay rr.sub.13, as shown. The leak current is prevented by closing only switch sw.sub.13.
The guard pipe 13 of each reed relay is ordinarily connected to the side of the relay which is closer to the D.C. power source 1 and the ammeter 3. That is, it is connected to the measuring device side. When an ordinary relay without a guard pipe 13 is used as the switch, a leak current possibly occurs in the switch. Although a circuit construction for preventing the leak current from affecting the measuring system can be designed by selecting a suitable combination of on-and-off states of the reed relays and the switches, the matrix device as shown in FIG. 3 has disadvantages because of connection of the matrix connection, as will be discussed in connection with FIGS. 4 and 5.
In FIG. 4, it is assumed that the reed relays rr.sub.11 and rr.sub.12 are respectively switched on and off and, at the same time the switches sw.sub.11 and sw.sub.12 are correspondingly switched on and off for measurement of a resistance value of the DUT 2. This arrangement provides opportunities to test a DUT under two different voltage sources or measuring devices. The guard pipe 13 of the reed relay rr.sub.12 does not contribute to the measurement of the DUT 2. The guard pipe 13 is connected to a guard line of a D.C. power source 1' which does not contribute to the measurement of the DUT 2. The guard line is kept at a ground potential. A leak current as indicated by an arrow in FIG. 4 still unfavorably occurs even if the relay rr.sub.12 and switch sw.sub.12 are open. This is because a potential difference exists between the signal line 43b of the reed relay rr.sub.12 at the side of the DUT 2 and the guard pipe 13 of the reed relay rr.sub.12. But the arrangement of FIG. 4 is entirely different from that of FIG. 3.
In order to overcome the problem, there has been conventionally proposed a technique as shown in FIG. 5. The relays rr.sub.11 and rr.sub.12 are respectively connected in series to new relays rr'.sub.11 and rr'.sub.12 to reduce the leak current. In this technique, respective guide pipes 13 of the added reed relays rr'.sub.11 and rr'.sub.12 are respectively connected to the guard lines 42b and 44b. The measuring system leak current as indicated by a dotted line hardly flows in its current passageway because the switch sw.sub.12 is open. Therefore, occurrence of a measurement error due to the leak current is substantially eliminated.
The device as shown in FIG. 5 has the following disadvantage of doubling the number of reed relays. Since the number of parts in the device is increased and the circuit construction is more complicated, the manufacturing cost is increased. At the same time, the reliability of the device is also reduced.
This invention as designed overcomes the above problem. Thus, an object of the invention is to provide an improved reed relay having a high guard effect, in which switching operations for connection between measuring devices and DUTs can be easily performed with a simple circuit construction. Another object is to provide a switch matrix device using the improved reed relay.